1. Technical Field of the Invention
The present invention relates generally to wireless communications systems, and more specifically, to mobile stations having multi-mode capabilities.
2. Description of Related Art
With the advent of wireless telecommunications systems, a number of different-protocols, procedures and systems have been developed to effectuate radio communications within wireless telecommunications networks. Wireless telecommunications systems, such as Advanced Mobile Phone System (AMPS), Time Division Multiple Access (TDMA), and Code Division Multiple Access (CDMA) systems are all examples of various wireless telecommunications standards and protocols. Within a given telecommunications system, such as CDMA, there may also exist a number of different protocols, frequencies, and parameters. Examples of such frequencies may include 800 MHz and 1900 MHz. Further examples of such protocols may include Personal Communications System (PCS) and Global System for Mobile (GSM).
Given the great number of wireless telecommunications service providers and respective service areas associated thereto, more and more geographic areas are being covered by multiple system providers. Each system within a particular geographic service area then provides an overlapping coverage with other system providers and competes for its own customers and service revenue. As an illustration, an AMPS service provider as well as a CDMA service provider may concurrently provide overlapping service coverage over a particular geographic area and compete for service revenue therein. Mobile subscribers, as a result, benefit from such multiple overlapping coverage by being able to choose from a number of different providers, service quality, and service plans.
With the development of telecommunications technology, mobile stations have also been improved wherein a single mobile station is capable of interfacing to and communicating over a number of different network systems using different protocols. For example, a mobile station capable of selectively communicating over two different telecommunications protocols is known as a “dual-mode” terminal. A dual-mode mobile station, for example, enables a mobile subscriber to communicate over a preferred or home service provider's communication system until the mobile station travels or roams outside of the respective coverage area. In response to a determination that the preferred communication system is no longer available, the dual mode mobile station then utilizes the available secondary or alternative telecommunications system with different protocol. By selectively switching over to the secondary telecommunications system, the dual mode mobile station is able to continuously provide radio communications service without interruption to its associated user.
A multi-mode mobile station typically provides such capability by maintaining a preferred provider or system list. When a mobile station is first turned on or initialized, the mobile station attempts to access the system identified as the most preferred system. As described, this identification may be made by referencing a system preference list maintained within the multi-mode mobile station. In response to a determination that the most preferred system is not available, the mobile station then attempts to access the next system identified by the system preference list. The mobile station repeats the above access processes until an acceptable system is allocated or the systems associated with the priority list are exhausted. A similar re-selection may occur when a mobile station already registered with a first telecommunications system travels into an area covered primary by a second telecommunications system. As a result, in response to a determination that the current signal quality associated with the first telecommunications system is falling below the acceptable threshold level, the mobile station then attempts to access the second telecommunications system.
Conventionally, a multi-mode mobile station monitors a particular message transmitted over a forward-link channel associated with the preferred service provider to determine its access status. As long as the mobile station is able to periodically receive that particular message within a predefined time period, the mobile station maintains its access to that particular service provider. As an illustration, within a CDMA system, the mobile station maintains its access to the preferred CDMA system as long as the mobile station receives a page channel (PCH) message every 3 seconds as required by T30m of Interim Standard 95 (IS-95). However, maintaining access to a particular service provider solely based on the number of particular forward link messages received within a given time period is inefficient and undesirable. There may be instances where even though the requisite PCH messages are periodically received from a CDMA service provider, the “health” of the overall system is not acceptable. Accordingly, the health of the system may not be properly determined by merely considering the number of messages received within a given time period. Consequently, only a few valid PCH messages would need to be received for a mobile station to continuously use the preferred system while another system (such as another CDMA or AMPS) is available and might be capable of providing better or more reliable telecommunications service. Maintaining access to such less desirable system while another more reliable system is available to service the mobile station causes undesirable and inefficient results. For example, such poor system condition increases the probability of failure for both mobile originated and terminated calls. It further causes poor handoffs to other systems.
Accordingly, there is a need for a mechanism to enable a multi-mode mobile station to more effectively select an optimal network system.